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suvlub t1_j4zotz4 wrote

There are actually humans with 44 chromosomes (22 pairs) walking around (typical human has 46 (23 pairs)).

The important thing to note is that these people have the exact same genes as anyone else, they're just organized differently - where other people have (2x)2 chromosomes, they have (2x)1 long fused one. Nothing is missing and nothing is extra, which sets them apart from people suffering from conditions like Down's.


CoolioMcCool t1_j4zrq2e wrote

Would they still be able to reproduce with people with 46 chromosomes?


suvlub t1_j4zu2gj wrote

Yes. Their children, however, would end up with 45 chromosomes, which would make it difficult, but not impossible, for them to reproduce. Their family has a long history of miscarriages, unfortunately.


Slashy1Slashy1 t1_j50e1rq wrote

Right, and that seems to illustrate OP's point. Having 44 chromosomes is obviously a pretty big fitness detriment, since it makes it harder to reproduce with other members of your own species. So how did a such a variation in chromosome numbers between species occur in the first place, if evolving it is a detriment?


thegreenrobby t1_j50mx28 wrote

Evolution isn't a perfect system. It's a game of repeated "good enoughs". If the genes with a disadvantage get a little lucky with their reproductive odds during the initial generations of the mutation, there's no reason a fully detrimental mutation might not stick around for a while.

Also, humans tend to be the exception to a lot of rules. Our knowledge of medicine (Edit: and agriculture, and a buncha other things) significantly alters our fitness odds, and allows many genes to reproduce that may not have otherwise survived.


HornedDiggitoe t1_j50sxod wrote

> Our knowledge of medicine significantly alters our fitness odds, and allows many genes to reproduce that may not have otherwise survived.

Knowledge of medicine can’t be credited for that. That was largely due to human knowledge of farming/agriculture, as well as human empathy to care for the weak. Humans not surviving long enough to reproduce was historically caused more by a lack of food than anything else. If you had a dead weight (disabled) human in your group and not enough food to go around, guess who isn’t going to get fed?


rising_ape t1_j50xhr4 wrote

Interestingly, our empathy "to care for the weak" may be more important than our knowledge of farming and agriculture here - we've found Neanderthal fossils that were severely disabled in life and would have been unable to care for themselves, but whose bones reveal that their initial injuries healed and that they lived on for years despite being "dead weight" (physically, at least).


HornedDiggitoe t1_j50xrqe wrote

Right, but that would’ve only been possible if the Neanderthals had enough extra food to feed themselves and the disabled Neanderthal. You don’t necessarily need agriculture to have an abundance of food, but it certainly helps a tonne to make food abundance widespread.

It still circles back to being about food in the end.


rm_systemd t1_j50yrtm wrote

It definitely can. Statistically, most people died in childhood from the many fevers, whooping cough, tuberculosis, syphilis, measles, polio, malaria, and infected wounds. There are even more tropical diseases, which is why Europeans had a life expectancy of 1 year in Central Africa prior to their discovery of Quinine.

Adult women then had to chance the maternal death rates due to hemorrhage and puperal fevers.

Those are the greatest reasons behind the 32 year life expectancy.

You can also credit sanitation, agriculture and industrialization, but vaccination soon after birth is mandatory for a reason, and that is why we had a way higher population than what ancient Rome and China could support, even with their excellent infrastructure and decent agricultural capacity.


HornedDiggitoe t1_j514owa wrote

Without food abundance none of that medicine would have helped much. All these medical marvels you brought up were invented after agriculture. Imagine what the life expectancy was for disabled/sick people prior to an abundance of food.

Also, 32 years old is old enough to have reproduced and pass on genes. Life expectancy was much lower prior to agriculture.


rm_systemd t1_j52wsn7 wrote

In ancient China, food was not more abundant. In fact, everyone outside of the top 2% ate mostly unpolished grains and wild vegetables, and were usually about 5 feet tall due to poor nutrition. However, Chinese medicine was effective as preventative medicine and supportive treatment, and so the empirical evidence stands that their cities were historically the largest until the industrial revolution entered full swing.

Farming in China has been largely unchanged for the last 2600 years, they had very little arable land per capita and no access to the abundance of the sea like Japan does. Rice is also a luxury for most of history, and only a staple in the South. Northern China was fed on wheat, millet and sorghum etc., and the Yellow River is the area that the Han culture originated and thrived for most of history.

Your point about feeding the weak only applies to famine and war, in a time where death rates are already high. It won't be statistically significant then, because everyone would be hungry and weak, then the plague or a hostile army would come out of nowhere and flatten them anyway. In that case, survival was as much luck as it was rational decisions.

The family, tribe or clan was also the most important unit in all of history, and they always provided for the infirm. Even Neanderthal tribes have left behind evidence that they supported the disabled. Liberalism was significant, because it recognized the individual, where the traditional conservative only saw clans as the smallest unit. That is not how it worked for the longest time. If you were family, you just fed them, it was that simple


Emu1981 t1_j52xrle wrote

>Imagine what the life expectancy was for disabled/sick people prior to an abundance of food.

What makes you think that there was no abundance of food before the discovery of agriculture? Hunter gatherer groups tended to migrate around to follow the food over the seasons. Between this and the low populations it would have been pretty rare for the groups to go hungry over a long enough period of time for individuals to starve to death.

Agriculture and animal husbandry is what allowed for humans to settle down and to start multiplying like rabbits.


thegreenrobby t1_j50wgn3 wrote

Agreed. Reducing the complexity of the human experience to "medicine" was a bit reductive on my part, although it certainly plays a part.


docroberts t1_j510f46 wrote

Fertility is one of many factors in the evolutionary equasion. In evolutionary time human populations have been very scattered At the center of a slightly isolated population this lineage reproduces normally. On the periphery of the isolated population there are more miscarriages, but adequate reproduction for introgression of useful genes into the population. It's probable Neandertal/Sapiens hybrids and Denisovan/Sapiens hybrids were significantly less fertile, yet their genes made it into our pool. Surprisingly the ancestral trees of individual genes are often very different than the species tree.


off_the_cuff_mandate t1_j513a05 wrote

If the 44 chromosome people survive though, it would likely be without procreating with 46 chromosome people, which would cause them to gradually adapt differently from the 46 chromosome people and eventually become a separate species.


Slashy1Slashy1 t1_j51f42i wrote

But that requires a substantial population of 44 chromosome people to already exist, at least enough to avoid extreme inbreeding depression.


harbourwall t1_j52kumd wrote

Inbreeding becomes less dangerous the more common it is, and people in early tribal groups were a lot more closely related than today. Genetic differences between groups increases and all it takes is a bottleneck event to make the tribe of 44s the new standard number of chromosomes. Speciation through increased diversity between many groups of genetically similar individual, followed by selective or random culling of many of those groups.

That's a viable explanation of how we ended up with 46 instead of the 48 the other great apes have.


Claycrusher1 t1_j50jd7y wrote

Why would 45 chromosomes be a problem but not 44?


wheatgrass_feetgrass t1_j50jzld wrote

When you reproduce, your chromosomes are split in half. One gamete will end up with 22 and one with 23. This will only create a viable gamete if the one with 22 includes the extra long boy but more importantly, that the set with 23 doesn't include the extra long boy. Duplicate genes aren't any better. Most trisomy conditions are incompatible with life.


suvlub t1_j50la00 wrote

A sex cell has 1 copy of each chromosome. They are created by meiosis, i.e. a classical cell with 2 copies of each splits into 2 sex cells. During this process, each chromosome finds its buddy, so they split nicely and you end up with 1 copy of each, not random half. That would be bad.

In the person with 45 chromosomes (assuming this specific kind of mutation where 1 chromosome is fusion of 2), the combined chromosome pairs up with random one of the smaller ones, and the other is left without buddy. That's bad. If you are lucky, it ends up in the same cell as the other small chromosome. If you are not, it ends up in the other cell.

The article I linked in the first comment has nice pictures illustrating this.


yerfukkinbaws t1_j513zuo wrote

The two unfused chromosomes from the parent with 46 will usually both pair with the fused chromosome that came from the parent with 44. This is called a trivalent, instead of the usual bivalent that forms in meiosis. The pairing actually usually goes just fine since the genetic content is not changed and these chhromosome fusions usually involve chromosomes that only had one arm before (acrosomes). What this means is that it's not as random as all that. There is still a chance that separating the chromosomes can go wrong, but the offspring of people who've had a fusion of this type are usually not infertile, just reduced fertility sometimes. Often not even by much and many, many cases are believed to be undiagnosed since there's no "symptoms."


stickymaplesyrup t1_j50nlvg wrote

Okay, hear me out.

One of the ways we define whether or not species are different is if they can reproduce together, and if the offspring are also able to reproduce. This is why horses and donkeys are still different species even though they can have babies, ie mules, because mules are sterile.

What if these 44 chromosome people grew in number and could have kids together (non-incestuously, I don't know if there are multiple families with this condition)? And then those kids could have kids, and so on.

Would this be the origin of a new species of human?

It's fun to think about and consider.


ohheyitslaila t1_j515r2u wrote

You actually picked a really interesting set of animals for your example!!! I apologize if I explain anything poorly, I just know a little about this stuff because my family breeds horses. I feel like this kind of hits on what you’re asking.

So female horses and male donkeys can be bred to each other, that produces mules. Female mules have 63 chromosomes, which can’t usually be split evenly to produce a fertile egg. BUT, some female mules actually produce an egg that does have an even set of chromosomes. It’s just that the egg rarely meets up with a sperm with a matching set of chromosomes. A case of this incredible, one in a million chance did occur and a female mule gave birth to a male foal in 2007. The foal had some deformities, believed to be caused by the chromosomal issues, specifically a problem with its legs. But it lived until about 2010, when it slipped on ice and was badly injured, leading to him being humanely euthanized.

I wonder if that kind of thing could happen in humans. It would probably be just as rare, if not more so.


czyivn t1_j51pj1j wrote

Which perfectly explains why different species frequently have different numbers of chromosomes: It's part of how you get a new species.

Imagine a family of these 44 chromosome people dropped on a desert island with another family of 46 chromosome people. Breeding within a chromosome number group is likely to be more successful than outbreeding. Therefore, over time, a couple possibilities are likely.

  1. The two groups stop interbreeding much and instead carry on as two indepdendent groups which accumulate more independent mutations over time until they are completely infertile with each other.
  2. One of the two groups dies out.
  3. They heavily interbreed. This might result in both groups dying out if there aren't enough fertile individuals in successive generations.

Several scenarios could result, over time, with the emergence of a new population that's not interfertile with 46 chromosome humans. A new species.


Bax_Cadarn t1_j506f30 wrote

I would like to point out many women can have 45 chromosomes and people with 47 or 48 aren't unheard of either - Turner's and Klinefelter's.


wheatgrass_feetgrass t1_j50kn4u wrote

The sex chromosomes are unique though. The X chromosome is the only chromosome that is almost fully functional whether there's 1 or more copies. The Y chromosome is not necessary for life, though it does serve a function besides "make boy", as missing it or duplicating it is not a side effect free situation as in XO and XYY like you pointed out.


Gonjigz t1_j50wy3x wrote

This is a very specific phenomenon though. 47 chromosomes are almost always incompatible with survival to adulthood unless the extra chromosome is a sex chromosome or a 21, and I don’t think monosomy of any of the autosomes is compatible with life.


Bax_Cadarn t1_j50zfyj wrote

Nope, Turner's is the only nonlethal monosomy, but there are 2 more non-sexual trisomies that aren't lethal, 13 and 18 iirc.


Gonjigz t1_j5214z5 wrote

Edward’s and Patau syndrome, both of which have more than a 90% mortality rate before 1 year of age. There are extremely rare cases of survival beyond childhood which is why I said almost always, but by and large these syndromes do not allow for survival to adulthood.


Bbrhuft t1_j51ith5 wrote

Here's the reference:

>Robertsonian translocations occur in approximately one in every 1000 newborns. Although most Robertsonian translocation carriers are healthy and have a normal lifespan, they are at increased risk of spontaneous abortions and risk of producing unbalanced gametes and, therefore unbalanced offspring. Here we reported a previously undescribed Robertsonian translocation.

Song, J., Sun, L., Xu, S., Liu, N., Yao, Y., Liu, Z., Wang, W., Rong, H. and Wang, B., 2016. A family with Robertsonian translocation: a potential mechanism of speciation in Humans. Molecular Cytogenetics, 9(1), pp.1-7.


eloel- t1_j51afhx wrote

Also, zorses. Horses and zebras have wildly different counts, and yet we can get zorses.


minecon1776 t1_j51esw2 wrote

maybe this is what all that "unused" dna codons are for. It is to make when a chromosome splits, the odds that the broken part is in a vital gene very low.


fruticosa t1_j4zmaji wrote

The "how" is through "chromosomal mutations". These types of mutations occur at the chromosomal level (rather than small, single mutation that happen at the level of the nucleotide). For example, a chromosome can literally break in half at some stage during gametogenesis (the formation of eggs an sperm), turning one chromosome into two.

A common example in plants is through whole genome duplication. A plant starts with two copies of each chromosome, it undergoes some kind of mutation which duplicates all the chromosomes - now the plant has four copies of each chromosome. This is actually how plants (like strawberries) have evolved to become bigger and bigger, they have eight copies of each chromosome.

If some chromosomal duplication occurs, the organism now has redundancy in that it has more copies of each gene than it needs. Over time, the extra genes go through specialisation and evolve to become new genes with different properties (this is how gene families evolve).


dkysh t1_j506qis wrote

Another (close) example: Human's chromosome 2 is the result of the fusion of other 2 smaller chromosomes present in all other great apes, chromosomes 2a and 2b.

The content of chr2a+2b is almost identical to human's chr2, even with genes following the same order. This makes them much more compatible and probable to recombine and produce viable offspring.


VictoriousEgret t1_j5076sh wrote

How do we know this/find this out? Is it that someone noticed similarities between two chromosomes common in great apes and chromosome 2 in humans?


ikefalcon t1_j508nej wrote

As the previous commenter said, human chromosome 2 has nearly identical sequences to chromosomes 2a and 2b in the other great apes. It’s believed that human ancestors like Neanderthals had 23 chromosome pairs like humans.

Also, chromosomes usually have 1 centromere (center link between pairs of chromatids) and 2 telomeres (basically end caps of the chromosome). Human chromosome 2 has a vestigial (unused due to no longer being needed) extra centromere and 2 vestigial telomeres found inside the chromosome sequence.

This is pretty good evidence that there used to be 2 chromosomes before they fused together.


wheatgrass_feetgrass t1_j50lv6a wrote

>Human chromosome 2 has a vestigial (unused due to no longer being needed) extra centromere and 2 vestigial telomeres found inside the chromosome sequence.

Goddammit that's cool.

I can't be sure of it quite yet, but I have a feeling sequencing the entire genome is the single greatest human breakthrough in my lifetime. I was in middle school when my science teacher told us it had been done for the first time. When I was in college I toured a sequencing facility where it was being done for outrageous cost per sequence on machines bigger than my house. Last year I got my own DNA fully sequenced, every single base pair, for about two day's wage.

Reading your own ancient programming code letter by letter is a weird, almost disassociating experience.


dkysh t1_j50geul wrote

Building a "reference genome" for a species from scratch is a whole field of science.

When people do "normal sequencing", the genome is broken into an infinity of small pieces. The genome of chimpanzees, gorillas, and orangutans fit so well the human genome, that scientists usually use the human reference genome to study great apes.

Some scientists compared both the human and the chimpanzee reference genomes built independently from scratch, and they found just minimal differences. All this shows that the human chr2 and the great ape chr2a and 2b are almost identical in they just happened to fuse in proto-humans sometime in the last 6 million years.


JFSOCC t1_j50r6a8 wrote

So are people with down syndrome a form of speciation?


dkysh t1_j50ssc6 wrote

If we were bacteria, yeah, sure. But we are extremely complicated multicellular organisms. A whole genome trisomy screws up the balance of gene expression to such an extreme that most trisomies are simply lethal and never observed (they end up in miscarriage).

A gene fusion is a less drastic event, where 2 chromosomes happen to be connected, but the genetic load is identical.


radlibcountryfan t1_j50eao9 wrote

Just to be a bit pedantic, the new genes actually have three possible fates: becoming genes with new functions (neofunctionalization), becoming a part of the same pathway where both copies take on part of the work (subfunctionalization), or losing functionality entirely (psuedogenization).

Evolution is cool.


shadowyams t1_j50iajs wrote

Chordates (vertebrates and a few close relatives) also went through two rounds of whole genome duplication early on in their evolutionary history (this is the 2R hypothesis, the evidence for which is pretty solid at this point), so while it's not as common or well-tolerated in our clade compared to plants, it's still something that can happen occasionally.


SloeMoe t1_j50bvdg wrote

Sounds kind of like forking in software development. So let's say it's a sexually reproducing species. At gametogenesis, the chromosomes are doubled, how does that sperm with double chromosomes sync up with non-doubled eggs from any other individual in the population? Beyond that, how would any offspring find a suitable mate, seeing as they have dissimilar chromosome counts now?


deadcommand t1_j4z4aeh wrote

So this hits the pitfall many fall into of thinking evolution is a thing you choose. It’s not. Mutations, most of random, happen and the ones that are useful in some way, or at least not actively detrimental, get passed on.

The reality is that science still doesn’t really have good answers when it comes to how a species’ chromosome is divided up. Hell, there’s a lot about our own genetics that we still don’t understand. For example, introns are something of a mystery. We know what they are, we know what happens with them. But why? Still not sure.

So the answer to your question basically comes down to “we’re not entirely sure.”

Not satisfying, I know, but that’s science. The more you know, the more you realize you don’t know.


Vercassivelaunos t1_j4zwhgm wrote

I don't really see them falling into that pit. I read the question more in the following sense: If an individual randomly has additional or fewer chromosomes, and reproduction with all the other individuals having the standard number of chromosomes is difficult, then how come so many species did manage to reproduce after introducing or losing new chromosomes?


Painting_Agency t1_j50grma wrote

> “we’re not entirely sure.”

As someone with a moderate biological education I always assume "if the chances of something are low, just remember evolution has a LOT of time and a LOT of DNA replication events to work with" is the answer to weird questions about evolution.


deadcommand t1_j50l18n wrote

I both like and dislike that. Because on the one hand, yeah, you’re not wrong. On the other, it feels a bit like it discourages exploration as a kind of “yeah it just be like that” sort of thing.


Painting_Agency t1_j50w1ap wrote

The thing is that with science we CAN explore how things happen. There isn't really a "why" in evolution though. I mean, there is a why in the proximal sense, but there's no plan. Alleles either propagate in a population or they don't. With good data analysis we can map how that happened, when it happened, and we can postulate and test what causes it to happen.


anon525as t1_j4zidhj wrote

Idk about chromosomes part of this post's context. But adding to the evolution part for anyone who's interested.

When cells divide and grow the genetic code isn't copied 100% correctly. There are errors, some get fixed some don't. The ones that don't get fixed and continue to exist are what we call cancer.

Genetic mutation is a random thing that just happens when cell divide and grow because of not 100% being copied exactly the same. If there is a mutation which gives the individual an edge over other beings of the same species then it becomes easier for them to survive and reproduce. As their offspring increase and the ones with this mutation absent die or have less offsprings we see what we know as "nature's selection"


shitivseen t1_j4zrnbl wrote

To clarify: The vast majority of genetic mutations do not cause cancer. Only when the mutation alters the function of tumor suppressor or pro-oncogenic genes the risk of cancer increases. Additionally there are pathways cells can utilize to prevent cancer development even if a single one mutates. A rule of thumb is that six of these specific genes need to be mutated to actually cause cancer (this can vary of course).


PoopLogg t1_j4ztif9 wrote

> >When cells divide and grow the genetic code isn't copied 100% correctly. There are errors, some get fixed some don't. The ones that don't get fixed and continue to exist are what we call cancer. >

Sometimes it's a cancer, sometimes it's a webbing between your fingers, and that helps you glide when you fall off a tree, and in a million years you're a bat.


No_Ingenuity3366 t1_j4zp51q wrote

Your partly correct only cell devision DNA makes two exact copies that can repear themselves to 100%. When DNA is dammaged by toxic intake true food and environment DNA alters. RNA comes in 3 forms mRNA tRNA and rRNA with each their own fundamental function of the genome translation into practice. However when the genome(DNA) is dammaged the RNA takes over the error and can't fixt it so altered/dammaged proteins get build and altered processes get activated with auto-immuun disease and cancer as a consequence.Genetic mutation is not random it's bonded by toxic environment and ultraviolet light. All chemical elements not from nature in human body (foreign) is toxic and stacksup in organs which leads to disease.


PhantomSkyz t1_j4zwyp7 wrote

>genetic mutation is not random

You were doing really good up until this point... Then I stated rereading your whole post and I'm pretty confident the latter half is mostly inaccurate.

E: actually, besides the 3 types of RNA, this is entirely inaccurate.

E2: Whatever bs you just tried coming back with got auto hidden. The person you were referring to doesn't have peer reviewed studies on the topic anyway.

E3: idk why you keep getting auto hidden, likely poor behavior. Anyway, obviously RNA can't repair missing data, that's literally not it's job... Dunno why you tried to change the subject though.

E4: whelp, you're a lost cause, 3 comments and not a single one shows up.


InfernalOrgasm t1_j4ziyp1 wrote

And at the root of it all is this magical thing called quantum mechanics; wherein particles that don't even exist ... amount to observable, tangible, forces? It's really complicated


beezlebub33 t1_j4zurfq wrote

Your premise is incorrect. During reproduction, the chromosomes have to line up in order for them to produce offspring. The number of chromosomes is important but it's not clear exactly how difficult the chromosome number makes for reproduction, relative to other factors, and it depends on where and how the chromosome number changed. It is not the barrier to evolution that it is portrayed in anti-evolution literature.

People with Down's syndrome can and do reproduce and they have an extra chromosome. Consider Robertsonian translocations. which can reduce the number of chromosomes. See:

Of course, the most famous 'cross' is a male donkey and a female horse to produce a mule, which is sterile. However, there are a large number of equine species, and they have wildly different numbers of chromosomes. See: and . Some of the crosses are fertile; for example, Przewalski's horse (66 chromosomes) and domestic horses (64 chromosomes) can and do produce fertile offspring.

Scientists can study the changes that have occurred in the number chromosomes, their shape (lengths of arms for example), banding patterns, etc. (this is called the karyotype of the organism) in related to help understand the evolutionary history of them. See, for example: which focuses on equines. However, the same thing can be done for much more distant species. See this which reconstructs different chromosomes a wide diversity of animal: Figure 2 in particular shows how the chromosomes line up, and what happened as they split, merged, grew, and shrank.

Summary: reproduction between individuals with different numbers of chromosomes can and does happen. The history of related (both near and far) animals provides evidence for what changes occurred in chromosome number (and shape, and banding, etc.).


yerfukkinbaws t1_j514tql wrote

> During reproduction, the chromosomes have to line up in order for them to produce offspring.

Just to be clear, chromosomes do not line up during reproduction. They only pair during meiosis, which is the production of sperm or eggs, but not during fertilization or embryonic development (mitosis). So chromosome number has absolutely zero effect on whether two individuals can produce offspring together, but may affect the fertility of their offspring.

This confusion over when chromosome pairing happens leads to a lot of the misconceptions around chromosome number.


CoolFreeze23 t1_j51rx41 wrote

I think your misunderstanding OP's full question. They're asking, if the reproduction is difficult and the offspring are most likely sterile, how did species come to have different chromosomes at all? If we all have a common ancestor when you go back far enough, that must mean a mutation happened that caused one of them to have a different number of chromosomes. Most of the comments are saying how the actual producing offspring isnt difficult, but the fertility of that offspring is rare. But then how could those mutations in the number of chromosomes have become persistent enough that the offspring of them were fertile and able to even pass that down themselves?

Summary: If a member of a species was born with an extra chromosome, or two chromosomes fused, their offspring have a high change of being sterile. How could the increase of decrease of a chromosome become wide spread in a species if that happens?


AdeepinAmerica t1_j51xd67 wrote

People with 45 chromosomes (or to generalize to other species: individuals with mismatched chromosome counts as a result of evolutionarily recent chromosome fusion or splitting) do not generally have a high chance of being sterile. They may have a high chance of some kind of reduced fertility, though even that is not clear. There's a major detection bias here since almost no one ever gets karyotyped unless they believe they have fertility problems in the first place. This inflates estimates of how often these chromosome mismatches cause fertility problems.

The answer to the question of why any mutation that has any negative effect on fertility would spread is, as others have said, random success. In evolutionary terms, this is called genetic drift. Genetic drift is sometimes thought of as affecting neutral variation that doesn't have either positive or negative effects. However, it's been well understood from the beginning of genetic drift research that what really matters is the "strength" of genetic drift versus the "strength" of natural selection. Many things can make genetic drift stronger, like a small population or an expanding population or pops where some individuals reproduce more than others, etc. If enough of these drift exagerating factors are found in a population (as they often have to humans), then even variation with pretty strong negative effects can still spread. Beneficial mutations can also be lost in the same way.


beezlebub33 t1_j52d84h wrote

>If a member of a species was born with an extra chromosome, or two chromosomes fused, their offspring have a high change of being sterile. How could the increase of decrease of a chromosome become wide spread in a species if that happens?

I think I understood the question and answered it. 1. The sterility of an offspring with an additional / fused chromosome isn't that high as shown by examples, it can be neutral; and 2. neutral mutations can become fixed.

The argument is quite similar to mutations in general. There is the general opinion that mutations are bad and overwhelmingly deleterious. They aren't. Most are neutral; the result is most people have mutations, often quite a few. Those mutations can become fixed simply because there are so many of them and they are not selected out. There are certainly bad mutations, which cause developmental or functional problems. They are sometimes really bad and really obvious, and people remember those. Sometimes they are good and increase selection.

Similarly, sometimes chromosomes fuse or split, and it doesn't make a difference. Sure, sometimes, in fact more often than not, they are bad and get selected out. But sometimes they are neutral, and sometimes the different number gets fixed. This is not unexpected.


reedmore t1_j50ha2w wrote

I'm learning so much through this, thank you!


Sylvurphlame t1_j503s5f wrote

Difficult but not impossible. So with uncountable attempts at reproduction every generation you eventually get viable offspring with differing numbers of chromosomes that can branch off into their own species.

It’s a large numbers game.


Dro-Darsha t1_j50nokv wrote

It’s not enough to have one or few viable offspring though. Also they have no good way to find each other, so they need to be able to reproduce with the other chromosome count until they reach a critical number


CoolFreeze23 t1_j51r4ah wrote

I think some of the comments are misunderstanding OP's full question. They're asking, if the reproduction is difficult and the offspring are most likely sterile, how did species come to have different chromosomes at all? If we all have a common ancestor when you go back far enough, that must mean a mutation happened that caused one of them to have a different number of chromosomes. Most of the comments are saying how the actual producing offspring isnt difficult, but the fertility of that offspring is rare. But then how could those mutations in the number of chromosomes have become persistent enough that the offspring of them were fertile and able to even pass that down themselves?